Al-Qaida's Syrian affiliate and its allies seized the Syrian city of Idlib on Saturday from government forces for the first time in the country's civil war, according to Islamic fighters and a monitoring group.

The Britain-based Syrian Observatory for Human Rights said an alliance consisting of the al-Nusra Front and other Islamic groups took control of Idlib after clashing with Syrian forces for four days. The Observatory said seven fighters were killed, while many soldiers were killed or captured.

The al-Nusra Front, which is leading a group of ultra-conservative rebels, announced the capture of the northwestern Syrian city through social media.

The Observatory said rebel fighters seized control of most of Idlib in a push Friday evening and early Saturday after collapsing government forces withdrew to their bases and several other buildings in the city.

The Local Coordination Committees, another opposition activist collective in Syria, also reported the "almost complete" capture of large parts of Idlib by the rebels.

Government news agency

Syria's official news agency, SANA, did not report Idlib's fall, saying the army is fighting "fierce battles" to gain control of the city. The Observatory said Syrian warplanes are targeting Islamic fighters around the city.

The fall of Idlib would mark a major blow to Syrian President Bashar al-Assad's regime. It would be the second provincial capital and major urban center lost by Assad's forces in the four-year-old conflict.

Earlier this week, rebels, led by Nusra, captured the ancient and strategic town of Busra Sham in southern Syria.

The northeastern city of Raqqa fell to rebels including the al-Nusra Front in 2013. It has since been taken over by the Islamic State group, which considers Raqqa its main foothold in Syria.

At an Arab League summit in Egypt Saturday, U.N. Secretary-General Ban Ki-moon said he is angry and shamed by the failure of the international community to stop Syria's raging civil war.

More than 220,000 people have been killed in the conflict and millions of others have been forced out of their homes.